Most likely a conbination of the amount of sun exposure and other poaitive genetic mutations that happened alongside melanin levels.

An example of the complexity of genetic changes being intertwined is resistance to malaria and sickle cell anemia. The benefits of resistance to sickle cell anemia outweigh the negstives of the increased chance of sickle cell anemia so the mutation has persisted.

It is likely that North African populations had something that was beneficial as a tradeoff for their comparably lighter skin. Wearing clothing that covers a lot more of their body and having shelters from the sun could also help to mitigate some of the sun damage.

So it is complicated and most differences are due to a combination of genetic traits, they don’t get passed down one trait at a time.